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I replaced a two-wire battery powered tstat with an Hvac powered one (without battery back up). So the 'Common' line was unused until now.

After about 10 minutes of calling for heat, the tstat loses power and turns off and comes back on a minute or so later. It doesn't seem to be a tstat issue. last time it went dead, I pulled it off and checked the wires with a meter, it read ~3V or less for a while and came back up to normal.

This only happens during heat-call and not other times. What's going on here? Is there something that kicks in after a few minutes and chokes the tstat?

From your wiring description, I'm thinking you're in over your head.
We can't give DIY advice.
I would call for service.

"Hey Lama, hey, how about a little something, you know, for the effort." And he says, "there won't be any money, but when you die, on your deathbed, you will receive total consciousness." So I got that goin' for me, which is nice. - Carl Spackler

lol, how ironic, I earn my living designing hvac products. Figured that'll qualify as not being in over my head. Should I have included a schematic diagram? pulled the wiring the labels from the Gas Valve. Here's a picture.

It is a bit confusing for me, I haven't worked with too many of the old-school boilers, hence the question.

Explaining what could be causing the loss of power to your new thermostat would be giving DIY information which isn't allowed by site rules.

I would also suggest that you call in someone proficient with boilers to take a look at your system. If the power to the thermstat is going away after only 10 minutes of the boiler running and then comes right back on you've got bigger problems that they should be easily able to find. While they're there, ask lots of questions and get schooled up on the "old-school" boilers.

Just curious, but what kind of HVAC equipment do you design?

Use the biggest hammer you like, pounding a square peg into a round hole does not equal a proper fit.

I see. So it was about DIY (now understand the meaning, thanks to you), not necessarily about the wiring diagram.

I'm afraid you were more helpful than intended (thanks, honestly). Turns out the High Limit on the aquastat was set to 160 which was kicking in and pulling the power out of the transformer. The old battery powered tstat must have ignored this for years.

Would asking "what is the best Aquastat High Limit setting?" still be DIY?

What I think is the problem is, is somehow with no air in the lines, valves open, and circulator pump running, the 100K BTU boiler can't get rid of the heat fast enough with only 4 rads. And if I can't see the 'simple thing to do' then tech call it is. I would have liked to wait and ask the tech to replace the unit with a hi-efficiency of the right size. This feels like asking a tech to fix a modem on a 386.

lol, how ironic, I earn my living designing hvac products. Figured that'll qualify as not being in over my head. Should I have included a schematic diagram? pulled the wiring the labels from the Gas Valve. Here's a picture.

It is a bit confusing for me, I haven't worked with too many of the old-school boilers, hence the question.

I think you misunderstood what he meant.

We have a vetting process here, and if you qualify as a Pro, you can post in the Pro section and get a lot of technical help.

We can't post what you need in open forums, and certainly not in the AOP section.